There has been a focus in the industry on developing new and improved olefin polymerization processes. Advances in polymerization processes and catalysis have resulted in the ability to produce many new polymers having improved physical and chemical properties useful in a wide variety of superior products and applications. With the development of new catalysts, the choice of polymerization type (e.g., solution, slurry, high pressure or gas phase) for producing a particular polymer has been greatly expanded. Illustrative of the advances in catalysis is the development of metallocene catalyst systems. Exemplary catalyst compounds for use in a process for polymerizing olefins and methods for preparing such catalyst compounds are described in, for example, U.S. Pat. Nos. 6,608,153, 6,472,342, 6,391,819, 6,306,984 and 6,300,436.
A polymerization process such as a gas phase (GP) process may exhibit melting and fusion of polymer particles as a result of the exothermic nature of the reactions involving metallocene and other catalyst compounds. Particularly with a fluid bed GP process, fused and agglomerated granular polymer particles may cause fouling and sheeting in the reactor environment causing disruption in continuity and ineffective operation of various reactor systems. Thus, reducing the fusion of polymer particles is a major objective for polyolefin producers.
Various techniques and compositions have been developed that are said to result in improved reactor operability. One method is to operate the reactor at lower temperatures than the Melt Initiation Temperature (MIT) of the polymer produced. Another method is to introduce an antistatic agent and/or continuity additive into a reactor before or during a fluid bed polymerization reaction to reduce sheeting and/or fouling in the reactor during polymerization. Various continuity additives used in polymerization processes are described in U.S. Pat. Nos. 6,482,903, 6,660,815, 6,306,984 and 6,300,436. Typically, a continuity additive is not catalytic but may be combined with a catalyst before or after being introduced into the reactor. An example of a continuity additive, such as a metal carboxylate, may be commonly utilized either on the catalyst or introduced separately in the reactor during polymerization. It is believed that the presence of metal carboxylates such as aluminum stearate (AlSt) on the catalyst or its proximity can prevent new polymer particles from agglomerating as the active catalyst site begins the polymerization process with heat generation, thereby preventing overheating and melting. As the polymer particles begin to melt, AlSt with its high melting point may act as a gelling agent to prevent reactor fouling and sheeting.
While the techniques and compositions previously described may result in some improvement in reactor operability, some are costly to employ and/or may exhibit other drawbacks. Operating the polymerization reactor at lower than desired temperatures, i.e., below the MIT, could cause significantly reduced catalyst production rates. The continuity additives previously mentioned may also cause a significant reduction in catalyst activity. AlSt, in particular, may exhibit low solubility and may gel when heated in hydrocarbons such as iso-pentane and hexanes at elevated temperatures. The gelled AlSt slurry exhibits high viscosity and may be difficult to handle.
In order to improve operability in reactors, oil slurry continuity additives may be pumped directly into the reactor when reacting with catalysts systems. The drawbacks with utilizing oil slurry continuity additives may include the relatively complex nature of the oil slurry preparation method wherein extra drying steps are needed for both oil and AlSt to reduce moisture content. Also, the transportation of oil slurry containers throughout the world may be costly. Furthermore, a mixing skid may be required to ensure that homogenous AlSt slurry is charged into the reactor. Another drawback with utilizing an oil slurry continuity additive is that it may cause a reduction in the activity of certain catalyst systems. Thus, there is still a need for compositions and processes that are effective in improving reactor operability during polymerization processes by reducing the fusion of polymer particles while maintaining the activity of the catalyst compounds.